This invention relates to shut-off valves for use in a downhole string of tools adapted to be retrieved from a well under pressure.
In completing a product recovery well, such as in the oil and gas industry, several downhole tasks or functions must generally be performed with tools lowered through the well pipe or casing. These tools may include, depending on the required tasks to be performed, perforating guns that ballistically produce holes in the well pipe wall to enable access to a target formation, bridge plug tools that install sealing plugs at a desired depth within the pipe, packer-setting tools that create a temporary seal about the tool and valves that are opened or closed.
Sometimes these tools are tubing-conveyed, e.g. lowered into the well bore on the end of multiple joints of tubing or a long metal tube or pipe from a coil, and activated by pressurizing the interior of the tubing. For strings of multiple hydraulically-activated tools, internal passages through upper tools along the string provide hydraulic communication between lower tools and the tubing. Such passages, particularly in perforating guns, may be breached by the operation of the tools and thereby exposed to well bore pressure and fluids. Sometimes such exposure is desirable to provide a path for circulating fluids down the tubing and out into the well bore as the tool is retrieved.
Often it is desirable to retrieve such tools with the well at an elevated pressure. Reducing the well head pressure to retrieve the tools (known as killing the well) can adversely affect subsequent well productivity. To retrieve tools under pressure, it is common to use a lubricator (a sealed stand pipe) above a blowout preventer (BOP; a well head bore seal). The tool string is pulled up into the lubricator under pressure, the blowout preventer is closed beneath the tool, and the lubricator pressure can then be bled off before removing the tool string. For tool strings which are internally sealed (e.g., do not have internal hydraulic passages which may be open to the well bore during retrieval), the blowout preventer (or similar sealing device) may be sealed about the outer diameter of the string below a joint between string sections, and the string sections removed one at a time.
Using this technique with strings having internal hydraulic passages open to the well bore, however, the maximum length of the tool string is generally limited to the length of the lubricator. The entire tool string is retrieved fully within the lubricator to form a seal by closing the blowout preventer, as sealing about the outer diameter of the tool string would not seal off well bore pressure because of the internal tool string passages.